In order to reveal the pathophysiology of photoparoxysmal responses (PPRs) in photosensitive patients with hereditary dentatorubral-pallidoluysian atrophy (DRPLA) who had expansion of the CAG repeat in the DRPLA gene, we studied the characteristics of PPRs using optical filters with specific wavelength transmission. In two patients, the wavelength spectrum around 700 nm (670-720 nm) was apparently the only visible range essential for eliciting PPRs, and flash lights containing the essential wavelength elicited PPRs. In another patient, PPRs were elicited by flash lights above certain quantity of light and independent of the wavelength composition of the lights. These data suggest that two different pathological conditions contribute to PPRs in DRPLA patients; one condition depends on the essential wavelength spectrum around 700 nm, and the other not on the wavelength, but on the quantity of light. The condition contributing to PPRs in all three patients was not determined directly by the level of the CAG repeat expansion in the DRPLA gene.